An optical waveguide was fabricated in a diaphragm of LiNbO₃ with 0.5mm thickness by using Titanium indiffusion method and the capability of the photo-elastic effect for the change of polarization of the transmitted light from the waveguide that is caused by applying external pressure was measured. For increasing of the change of polarization, the location of the waveguide was considered near the edge of the diaphragm and it has been shown that in spite of relatively large thickness of the diaphragm, the change of polarization for the waveguide near the edge of diaphragm is sufficiently great so that it can be used as a pressure sensor with accuracy better than 0.5 atm.